The present invention relates to wireless communications, and more particularly to an automatic gain control system and method for a wireless communication device configured in a zero intermediate frequency (ZIF) architecture that utilizes a DC control loop to enable direct conversion of radio frequency signals to baseband frequency and a gain procedure that more accurately determines and controls DC voltage levels.
The present invention is described in relation to a transceiver employed in a wireless local area network (WLAN) configuration. It is understood, however, that the present invention is not limited to WLAN configurations, but instead may be employed in other types of radio or wireless communications for other types of applications. A Zero Intermediate Frequency (ZIF) architecture is a wireless transceiver implementation that is utilized to obtain sufficient performance and higher data throughput at lower cost and power consumption. This is particularly advantageous to the consumer market which demands multimedia and DVD applications requiring relatively good performance. The consumer market also demands a lower cost. The ZIF architecture is one way to achieve lower cost with sufficient performance by eliminating IF components and external filters. The target carrier frequencies are in the GHz range such as 2-5 GHz ranges and higher, although the present invention is not limited to any particular frequency range. The 2-5 GHz bands are relatively noisy with significant amounts of interference. The ZIF architecture is utilized to maintain a level of performance in a noisy environment.
In wireless receiver implementations, including ZIF architectures, variable gain radio front ends are used to enable reception of the range of signal levels possible in the system. An Automatic Gain Control (AGC) procedure is often used to control the gain of a baseband AGC amplifier in response to incident signals to achieve a target gain level for baseband (BB) processing. The AGC procedure and the radio parameters are first configured to enable the analog to digital converter (ADC) within the baseband processor to see the noise floor of the wireless medium. The AGC procedure then detects and locks onto transmitted signals, where the gain parameters are changed so that an incoming signal larger than the noise floor can be seen by the ADC without undue impairments. At the end of the signal, the radio gain parameters are then re-configured so that the noise floor is again visible to the ADC. These relatively simple actions become more complicated when operating with a ZIF architecture. While many possible configurations exist for radio front ends, the ZIF architecture is attractive due to its low component count. ZIF architectures, however, suffer from excessive DC components and DC can be adversely impacted by poor AGC control. Because the DC voltage levels can become excessively large in the ZIF radio, the signals of interest become invisible and difficult to detect and acquire.
The primary problem with the ZIF architecture is the development of DC offsets at baseband that degrade signal-to-noise ratio (SNR), which is directly related to performance of the system. The IF stage, which may be utilized to combat a significant level of DC offset, is not provided in the ZIF architecture. A number of sources of DC offset are due to many factors, including variations in circuit matching, and temperature drifts or changes caused by die self-heating of integrated circuits. All of the sources of DC offset are referenced to the input of the baseband amplifier in the baseband signal path of the receive signal processing chain. The gain range of the baseband amplifier must be sufficient to guarantee acceptable performance in a variety of environments. The gain range of traditional baseband amplifiers has been configured to operate at gain levels over 50 dB, for example, which was believed necessary to obtain the desired operating range. It has been determined, however, that at such high gains (50 dB or more), increasing the gain by 10 dB may increase DC from 500 millivolts (mV) to 5 V, which is an increase of over 4 V. The ADC, however, is generally limited to a relatively small voltage range (e.g., approximately 0.5 V), so that excessive DC overwhelms the loop and causes instability and potential operation failure.
It is noted that one possible solution of removing DC offset is through AC coupling, such as using coupling capacitors or the like. However, AC coupling results in filtering that tends to filter out low frequency content. The amount of data thrown away becomes significant above 1 kHz, so that AC coupling must only filter below 1 kHz. Within this range, however, the settling time is too long, such as on the order of approximately 100 microseconds (xcexcs), which is not practical in a bursty environment such as packet-based communications. The settling time is limited due to the bursty nature of communication. Short preamble time lines of IEEE 802.11a, b and g standards have little room for accurate DC estimation.
Another possible solution is to measure DC, and if larger than a predetermined level associated with ADC full scale (e.g., 500 mV), remove the full scale amount and let the correction settle through the radio. This procedure is repeated as often as necessary until the DC level is reduced and the signal is visible at the output of the ADC. While this appears to be a viable solution and has been used in the past, it required a significant amount of time. The iterative solution just described required as much as 20 microseconds (xcexcs) or more in many configurations. For an 802.11a implementation, however, the maximum allowed time to measure and eliminate DC is about 5 to 6 xcexcs. The AGC procedure must be configured to conserve time, which is a valuable commodity in a wireless transceiver.
It is desired to provide low cost and low power wireless communication devices for any type of wireless system and any type of application. The system must be relatively robust with significant performance and be capable of significant data throughput, including the higher data throughputs associated with newer or otherwise faster wireless standards, such as, for example, the 802.11a and 802.11g standards for WLAN communications.
A method of controlling amplification of a signal received by a ZIF radio having a power level within a predetermined full power range relative to a predetermined minimum noise floor according to an embodiment of the present invention includes amplifying the received signal using a baseband amplifier with a plurality of gain settings, converting the received signal to a digital signal using an analog to digital converter (ADC), and controlling the amplifying of the received signal for tracking noise floor and for attempting to acquire the received signal while limiting DC change within an available DC budget of the ADC. The controlling includes obtaining actual noise floor level by setting gain up to a maximum gain level within a first story power range at a lower end of the full power range sufficient for the ADC to view the minimum noise floor, detecting an overload condition and switching gain in a single gain step to within a third story power range at an upper end of the full power range sufficient for the ADC to view the received signal, and detecting a saturation condition of the ADC and switching gain in a single gain step to within a second story power range between the first and second story power ranges sufficient for the ADC to view the received signal.
The obtaining actual noise floor level may include switching the baseband amplifier up to a predetermined maximum gain setting using limited gain stepping to avoid exceeding the DC budget of the ADC. The method may further include settling after each of the switching, measuring DC of the digital signal and subtracting measured DC from the received signal, and after a final of the switching and settling, measuring power level of the digital signal.
The method may further include detecting a signal trigger condition in which power level is increased over the actual noise floor by at least a predetermined signal trigger threshold, measuring power level and DC level of the digital signal in response to detecting any one of the overload, saturation and signal trigger conditions, switching the baseband amplifier and subtracting DC from the received signal and settling if measured power level of the digital signal is greater than a predetermined target back-off power level of the ADC, verifying power level and measuring DC level of the digital signal, and subtracting any remaining DC offset from the received signal and settling.
The method may further include digitally amplifying the digital signal to achieve a power level equivalent to the target back-off power level of the ADC. The method may further include using an RF amplifier with high and low gain settings set to high gain for weak signals and switched to low gain for strong signals.
A ZIF radio for detecting an RF signal within a predetermined power spectrum relative to a predetermined minimum noise floor according to an embodiment of the present invention includes a ZIF receiver front end, an overload detector, an ADC, a saturation detector, a DC and power estimator, and control logic. The ZIF receiver front end converts the RF signal to a baseband signal and includes a baseband amplifier. The overload detector detects an overload condition of the ZIF receiver front end in which the RF signal is within an upper power range of a predetermined power spectrum and asserts an overload signal indicative thereof. The ADC converts the baseband signal to a digital baseband signal. The saturation detector detects a saturation condition of the ADC in which the RF signal is within a middle power range of the power spectrum and provides a saturation signal indicative thereof. The DC and power estimator estimates DC and power level of the digital baseband signal and provides estimation signals indicative thereof. The control logic limits gain of the baseband amplifier to a maximum gain setting sufficient for the ADC to view a lower power story including the minimum noise floor of the power spectrum, monitors the saturation, overload and estimation signals, and switches gain of the baseband amplifier once to place the received signal within view of the ADC in the event of either one of the overload and saturation conditions.
A processing system with wireless communications according to an embodiment of the present invention includes a processor, a memory, and a ZIF transceiver that detects RF signals within a predetermined full power spectrum. The ZIF transceiver includes a ZIF receiver front end, an ADC, a saturation detector, an overload detector, a joint DC and power estimator, and AGC/DC control logic.